1. Field of the Invention
This invention relates to sodium sulphur cells and is concerned more particularly with the cathode current collector in such a cell.
In a sodium sulphur cell a solid electrolyte, for example a beta-alumina ceramic, separates an anodic region containing sodium, which is liquid at the operating temperature of the cell, from a cathodic region which, at the operating temperature, contains liquid sulphur and sodium polysulphides. During discharge of the cell sodium ions pass through the electrolyte forming sodium polysulphides. The cathodic reactant comprising the sulphur and polysulphides has a high electrical resistance and it is the practice to put a matrix packing of conductive material, for example carbon or graphite felt, in the cathodic region to provide electronic conduction. Such graphite felt in itself has a relatively high resistance and it is desirable to keep the electronic current path through this material to the minimum length. For that reason, this matrix material is arranged to extend between the surface of the electrolyte material and an adjacent surface of a current collector. In a cell design where the sulphur is contained inside a tube of electrolyte material, this current collector may be a rod or composite rod disposed axially inside the electrolyte tube. The current collector rod has to provide a high conductivity path between the matrix packing material and an external terminal of the cell. It must, however, be chemically and electrochemically inert to the cathodic reactant material. Metals, such as aluminium and copper, are sufficiently conductive, but aluminium is passivated and copper is corroded after a few charge and discharge cycles of the cell. Stainless steel, which is much less conductive than aluminium or copper, is also corroded after a few charge/discharge cycles. For various reasons such corrosion causes a deterioration of cell characteristic. The mechanism of the deterioration is not completely understood and is probably different for each material. Certain nickel based alloys, in particular Inconel (a nickel alloy containing some 13% Cr 6% Fe by weight together with minor constituents), have longer lives.
2. Prior Art
For this reason carbon, particularly in its graphitic form, has been used as a current collector in many laboratory cells. Carbon in itself, however, is not sufficiently conductive to be used alone in long cells, which are operated at useful rates of charge and discharge. Other materials, which are thermodynamically stable in sulphur and polysulphides to the anode and cathode conditions that prevail in a sodium sulphur cell, have been proposed. A ceramic formed of oxides, such as tantalum or niobium doped rutile, is an example of such a thermodynamically stable material that can be used. Nickel oxide doped with lithium oxide and lanthanium strontium cobaltite may be used. Conductive ceramics are described in British Patent Specification No. 1,471,914. However such materials are not sufficiently conductive to be used alone in large cells, which operate at useful rates of charge and discharge.
It has, therefore, been proposed to use composite current collectors. U.S. Pat. No. 3,982,959 describes the use, in a sodium sulphur cell, of a cathode current collector of rod-like form arranged axially inside a solid electrolyte tube containing the cathodic reactant and packing material, this current collector comprising a carbon tube with a metal core. It is necessary to provide a deformable electronically-conductive interfere in the annular region between the carbon tube and the metal core in order to ensure good electrical contact across this region despite temperature cycling of the cell. Composites of this nature do raise manufacturing problems and it is difficult to construct such composites to have a useful life in a sodium sulphur cell. It would be preferable to have the corrosion-resistant material as a protective layer on a conductive substrate, which has other useful characteristics, such as being readily formed by drawing or rolling, readily weldable or having an expansion coefficient well matched to that of the ceramic electrolyte. Molybdenum is a corrosion-resistant material that is conductive but it is deficient in some of these useful properties mentioned above. It is sometimes convenient to use a protective coating of molybdenum on a substrate. It appears that molybdenum is corrosion-resistant in the cathodic reactant of a sodium sulphur cell because of the formation of a stable surface film of molybdenum disulphide, which is also electronically conductive.
Claims have been made for the efficacy of simple coatings of carbon carried out by the application of colloidal suspensions and of molybdenum coatings by the method of plasma spraying on substrates of steel, aluminium or iron-nickel-cobalt alloys, but those skilled in the art know that such coatings have only transitory efficacy in the cathode electrodes of the devices previously described. Plasma-sprayed molybdenum can be made to adhere to aluminium but the deposit is porous and the corroding species are able to penetrate through the porous layer to attack the substrate. In the case of a current collector formed by plasma spraying an aluminium rod with molybdenum, the resistance of a sodium sulphur cell containing such a current collector in the cathode increases enormously over about 20 charge/discharge cycles and is not therefore suitable for commercial cells.
It has also been suggested that a stable cell resistance can be achieved if a conductive substrate of aluminium is protected by a polyphenylene resin impregnated with carbon to confer the marginal conducting properties required in the corrosion-resistant sheath of the composite current collecting member. It is known, however, that the beneficial results of such a composite are much too transitory to find use in a practical energy conversion device.